The distinctive feature of "woodpecker" drilling machines is the repetitive withdrawal of the cutting tool from the hole to clear the chips produced in the drilling operation. All peck drilling machines repetitively perform the steps of feeding the drill into the work, retracting the drill to clear the chips formed by the cut material, and again feeding the drill into the hole made in the work to drill to a greater depth.
Drilling holes that are relatively deep is usually a problem when the depth of the hole is seven times or more greater than the diameter of the hole. The problems encountered when drilling beyond such depths are: (1) the hole tends to run out because of bending of the drill as it penetrates to greater depths, (2) drill life is shortened because more rapid dulling of the cutting edges occurs which increases the frequency of sharpening the drill, and (3) breaking of the drill. The problems are caused by the chips that pack in the flutes of the drill. When chip packing is eliminated, a properly sharpened drill can make a hole to very great depths without difficulty. It is well known that metal removal rates in drilling can be increased and dulling of the cutting tool can be reduced by using coolants during the drilling operation. The lubricity of coolants helps the chips move up the flutes of the drill and thereby counteracts to some extent the tendency of the chips to pack in the flutes. The only satisfactory and positive way to eliminate chip packing, however, is to intermittently withdraw the drill from the hole to cause the chips to be pulled out of the hole along with the drill.
In peck drilling, after the drill is withdrawn to clear the chips, the drill is usually rapidly advanced into the hole. As the drill approaches the bottom of the hole, the advance of the drill is changed to a slower speed suitable for recommencing drilling of the work. If the drill is allowed to strike the bottom of the hole on the rapid advance, the drill may break or be damaged. If the cutting tool does not break, then any damage to the drill is likely to make the hole run out by drilling crooked. Consequently, experience has shown that in peck drilling, the drill must not be allowed to hit the bottom of the hole in a rapid advance. Experience has also shown that when a hole is made completely through a work, the drill tends to break if it is allowed to jump forward as it breaks through the bottom of the hole. Consequently, at the breakout, it is provident to keep the feed rate at the same rate used in drilling into the work.
Air-hydraulic peck drills are well known and have been used for many years in drilling operations. Air-hydraulic peck drills, in comparison to the invention herein disclosed require comparatively long and tedious set-up procedures. Moreover, the conventional hydraulic machine may require readjustment as it warms up. Further, it is extremely difficult to consistently meter hydraulic fluid in the very small quantities required to control the drill feed when drilling small holes. The conventional hydraulic peck drilling machine employs a dashpot mechanism to control the rate at which the drill is fed into the work. Those hydraulic machines always let the feed rate speed up at breakout because the reduced resistance to the advance of the drill is not compensated by an offsetting reduction in the pressure on the dashpot which controls the feed rate.